That's very true, a manager must know how to speak, when you open your mouth you tell the world who you are, as a manager you are expected to choose your words correctly.A manager should know well how to speak with people, how to convince them to do things and how to punish them when it's needed. A manager should also be tougher than a regular employee because he has to introduce discipline at the employees and he also has to deal with tougher situations because he is the manager he is the ones who manage stuff and who has more important and essential responsibility for both the job and the employees. A manager should be a person who is never too late for the job, and he cannot leave the job too early because he has a greater responsibility for the job. And he definitely should be calm when stressful things happen as you say because he is the one who has to deal with them so it is very important.